1. Field of the Invention
The present invention relates to a scanning unit of a laser printer and a magnetic bearing apparatus therein, more particularly, to a scanning unit of a laser printer in which a cover is provided on a scanning motor so that the scanning motor is isolated from an outside and an inner portion of the cover is maintained in a vacuum state and in which the scanning motor is provided with a magnetic bearing having a repulsive force in thrust and radial directions, and to the magnetic bearing apparatus used in the scanning unit.
2. Description of the Related Art
Generally, a laser printer used in a laser component and a photo scanning and recording apparatus, etc. employs a semiconductor laser. In the laser printer, a light source or laser beam is irradiated to a rotating polygon mirror so as to scan a recording face formed on a hologram disk.
At this time, it a refracting speed of the laser beam by each face of the polygon mirror is increased, the scanning speed can be also increased.
FIG. 1 shows a structure of a scanning unit of a conventional laser print.
As shown in FIG. 1, the scanning unit of the conventional laser print comprises a semiconductor laser diode 100 for irradiating a laser beam as a light source of the laser printer, a collimator lens 200, a cylindrical lens 300, a polygon mirror 400, a scanning motor 500, lenses 600 for forming an image, a reflecting mirror 700, a horizontal synchronizing mirror 900 and a light sensor 1000. The collimator lens 200 makes the laser beam irradiated from the semiconductor laser diode 100 be parallel with an optical axis. The cylindrical lens 300 converts the parallel light passed through the collimator lens 200 into a linear light which is in a horizontal direction with respect to a sub-irradiation direction. The polygon mirror 400 moves the linear light at a constant linear velocity to perform a scanning operation. Therefore, the linear light has a negative refractive index with respect to the optical axis. The scanning motor 500 rotates the polygon mirror 400 at a constant velocity. The lenses 600 for forming an image polarize the linear light reflected from the polygon mirror 400 in a main scanning direction so as to compensate a spherical aberration of the lenses and focus on a scanning face. The reflecting mirror 700 reflects perpendicularly the laser beam passed through the lenses 600 so as to form the image on a surface of a photosensitive drum 600. The horizontal synchronizing mirror 900 reflects horizontally the laser beam passed through the lenses 600. The light sensor 1000 receives the laser beam reflected by the horizontal synchronizing mirror 900 so as to synchronize a point of time of forming the image on the photosensitive drum 800 with a point of time of transferring a printing data.
The lenses 600 include a spherical lens 610 for compensating the spherical aberration and a toric lens 620. The spherical lens 610 concentrates and polarizes the laser beam refracted by the polygon mirror 400. The toric lens 620 polarizes the laser beam in the main scanning direction, in which the spherical aberration is compensated by the spherical lens 610.
The operation of the conventional semiconductor laser scanning unit is described more fully.
If the laser beam as a light source is irradiated from the semiconductor laser diode 100, the laser beam is adjusted to be parallel with respect Lo the optical axis by the collimator lens 200. The laser beam passed through the collimator lens 200 is converted into the linear light in the horizontal direction with respect to the sub-irradiation direction by the cylindrical lens 300. The linear light passed through the cylindrical lens 300 is moved at a constant linear velocity by the polygon mirror 400 which is rotatably mounted on a rotating shaft of the scanning motor 500 to be rotated at a constant angular velocity, and forms an image of a point shape on the surface of the photosensitive drum 800.
That is, if the linear light of the horizontal direction is transferred to the polygon mirror 400, since the polygon mirror 400 is rotated at the constant angular velocity by the scanning motor 500, the linear light is refracted and moved at the constant linear velocity according to the refracting angle of the polygon mirror 400 and is concentrated by the lenses 600 for forming the image. The lenses 600 compensate an error of the spherical aberration fxcex8 and polarize the concentrated light to the main scanning direction, wherein f is a focal distance and xcex8 is a scanning angle. Then, the laser beam passed through the lenses 600 is perpendicularly refracted by the refracting mirror 700 so as to form the point shape image on the surface of the photosensitive drum 800.
Meanwhile, the scanning motor 500 for rotating the polygon mirror 400 simultaneously supports a radial load and a thrust load, and a half-spherical bearing device which is a fluid bearing device is employed in the scanning motor 500. Referring to FIG. 2, the scanning motor 500 is described more fully.
The scanning motor using the half-spherical bearing device comprises a fixed shaft 510 which is a rotating center of the polygon mirror 400, half-spherical bearings 520, 521 through which the fixed shaft 510 is inserted, a bush 530 for supporting the radial and thrust loads, a motor rotor 540, a motor stator 541, a hub 550 and a housing 560.
The fixed shaft 510 is inserted in the housing 560. The hub 550 is provided on an outer peripheral surface so that the polygon mirror 400 and the motor rotor 540 can be mounted thereon. The motor stator 541 is disposed apart from the motor rotor 540 at an interval.
The bush 530 for supporting the radial and thrust loads of the half-spherical bearings 520, 521 is provided with a through hole at the center thereof. The through hole has a larger diameter than that of the fixed shaft 510. Half-spherical grooves 531, 532 having the same radius of curvatures as those of the half-spherical bearings 520, 521 are defined on both ends of the bush 530. Further, a spacer 570 is provided in the through hole of the bush 530 in order to adjust a clearance between the half-spherical bearing 520, 521 and the half-spherical grooves 531, 532.
In the scanning motor as described above, when a power source is applied to the motor rotor 540 and the motor stator 541 and the motor rotor 540 and the motor stator 541 are rotated, the lower half-spherical groove 531 of the bush 530 is moved downward by a load applied to the bush 530 and is closely contacted with the lower half-spherical bearing 520.
At this time, if the lower half-spherical groove 531 of the bush 530 is closely contacted with the lower half-spherical bearing 520, the clearance(xcexcm) is defined between the upper half-spherical groove 532 and the upper half-spherical bearing 521. Therefore, the clearance between the upper half-spherical groove 532 and the upper half-spherical bearing 521 is lager than that between the lower half-spherical groove 531 and the lower half-spherical bearing 520.
Meanwhile, a plurality of spiral grooves are formed on the outer face of each half-spherical bearing 520, 521. If the bush 530 is rotated, a dynamic pressure is generated by air flowed in the spiral grooves. At this time, the dynamic pressure generated in the lower half-spherical bearing 520 is greater than that generated in the upper half-spherical bearing 521. Therefore, the bush 530 is risen upwardly by the dynamic pressure.
However, if the bush 530 is risen, upwardly, the clearance between the lower half-spherical groove 531 and the lower half-spherical bearing 520 is gradually increased. On the contrary, the clearance between the upper half-spherical groove 532 and the upper half-spherical bearing 521 is gradually decreased, whereby the dynamic pressure between the upper half-spherical groove 532 and the upper half-spherical bearing 521 is gradually increased.
The upper and lower clearances is varied for a while according to the movement of the bush 530. Finally, the bush 530 recovers a balance at a rotating place where the difference between the upper and lower dynamic pressures is the same as the weight of the bush 530.
However, in the conventional scanning unit as described above, there is a problem that, since the scanning unit employs an air pressure type bearing, a noise is generated when the scanning motor is rotated at a high speed.
Further, since the components of the scanning motor are so many, it is difficult to simplify the structure of the scanning unit.
It is therefore an object of the present invention to provide a scanning unit using a magnetic bearing, which can lower the noise according to the rotation of the scanning motor.
The other object of the present invention is to provide a scanning unit the structure of which is simplified, thereby miniaturizing the product.
To achieve the above objects and other advantages, there is provided a scanning unit of a laser printer comprising a semiconductor laser diode for irradiating a laser beam; a collimator lens for making the laser beam irradiated from the semiconductor laser diode to be parallel with an optical axis; a cylindrical lens for converting the parallel light passed through the collimator lens into a linear light which is in a horizontal direction with respect to a sub-irradiation direction; a polygon mirror for moving the linear light passed through the cylindrical lens at a constant linear velocity to perform a scanning operation; a scanning motor for rotating the polygon mirror at a constant velocity; a lens for forming an image, which polarizes the linear light reflected by the polygon mirror in a main scanning direction and compensates a spherical aberration and focuses on a scanning face; a reflecting mirror for reflecting perpendicularly the laser beam passed through the lens for forming an image so as to form the image on a surface of a photosensitive drum; a horizontal synchronizing mirror for reflecting horizontally the laser beam passed through the lens; a light sensor for receiving the laser beam reflected by the horizontal synchronizing mirror so as to synchronize a point or time of forming the image on the photosensitive drum with a point of time of transferring a printing data; and a cover for covering the polygon mirror and scanning motor, wherein the scanning motor is a magnetic bearing type motor.
Preferably, the cylindrical lens is integrally formed on the cover which is in an incidence course of the laser beam.
Preferably, wherein the lens for forming an image is integrally formed on the cover which is in the irradiation course of the laser beam.
Further, it is preferable that an inner portion of the cover is maintained in a vacuum state.
In addition, the scanning motor comprises a fixed shaft which is a rotating center of the polygon mirror; a housing through which the fixed shaft is inserted; a fixed thrust magnet which is fixed on an outer face of the fixed shaft; a fixed radial magnet which is fixed on an upper face of the housing; a hub on which the polygon mirror is mounted; a rotating thrust magnet and a rotating radial magnet which are respectively mounted in the hub corresponding to the fixed thrust and radial magnet so that the hub maintains a constant clearance with respect to the housing and the fixed shaft; and a motor stator arid a motor rotor for driving the polygon mirror.
Further, the rotating thrust magnet corresponding to the fixed thrust magnet is disposed so that the poles of the rotating thrust magnet respectively correspond to the opposite poles of the fixed thrust magnet, and the rotating radial magnet corresponding to the fixed radial magnet is disposed so that the poles of the rotating radial magnet are respectively corresponded to the opposite poles of the fixed radial magnet.